It’s no secret that as a population, Americans been getting heavier, but researchers now say that our weight problem may be worse than we thought.

In a study published in the journal PLoS One, lead author Dr. Eric Braverman, president of the nonprofit Path Foundation in New York City, which supports brain research, says that our current measure of obesity — body mass index, or BMI — significantly underestimates the number of people, especially women, who are obese.

Braverman and his co-author, Dr. Nirav Shah, New York State’s Commissioner of Health, studied 1,400 men and women, comparing their BMI measurement to their percentage of body fat, as measured by a dual-energy X-ray absorptiometry (DEXA) scan. While BMI is a simple ratio of a person’s height and weight, the DEXA scan — which is normally used to measure bone density — can distinguish between bone, fat and muscle mass.

Among the study participants, about half of women who were not classified as obese according to their BMI actually were obese when their body fat percentage was taken into account. Among the men, in contrast, about a quarter of obese men had been missed by BMI. Further, a quarter who were categorized as obese by BMI were not considered obese based on their body fat percentage. Overall, about 39% of participants who were classified as overweight by their BMI were actually obese, according to their percent body fat.

BMI is possibly more misleading for women than for men, Braverman and Shah say, since women lose muscle and bone, and replace it with fat, faster than men. BMI measurements don’t take into account precisely how weight is distributed in the body, and, as Braverman explained to HealthDay, “it’s the percent of body fat, not BMI, that makes you obese.”

Based on BMI, about one-third of Americans are considered obese, but when other methods of measuring obesity are used, that number may be closer to 60%, according to Braverman.

Physicians have complained for years that BMI is an imperfect measure of healthy weight. Extremely muscular people, for example, may weigh “too much” for their height, since dense muscle mass weighs more than fat, thus qualifying as obese even if their bodies contain very little fat. Yet it’s not extra weight itself, but excess fat, that leads to health problems ranging from hypertension and heart disease to diabetes and stroke.

“People aren’t being diagnosed [as obese], so they’re not being told about their risk of disease or being given instruction on how to improve their health,” Braverman told Health.com. Data show that people who start to put on pounds are more likely to continue getting heavier, increasing their risk for a number of chronic diseases.

So why is BMI still being used, if it’s not precise? For now, it’s the best and easiest way for clinicians to gauge a person’s healthy weight while taking into account his or her general body structure. (DEXA scans are far too expensive to be used as a routine measure during doctor’s visits.) BMI isn’t perfect, but many experts say it’s the best we have.

Still, as results like Braverman’s continue to build up, it may be time to consider other ways of tracking weight, and in particular, body fat. “It’s important to point out the weakness of the BMI,” Dr. Richard Bergman, director of Cedars Sinai’s Obesity and Diabetes Research Institute in Los Angeles, told the Los Angeles Times. “It’s a poor measure of fatness, and we do need better measures.”

In recent years, some experts have looked at using waist circumference as a measure of weight gain and body fat composition, and some studies show it may be more accurate than BMI. But those investigations are still ongoing, and BMI has the advantage of being used since the 19th century, with more than a hundred years of studies and data supporting its general usefulness in determining who is overweight or obese.

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.